Measurement of Formic Acid, Acetic Acid and Hydroxyacetaldehyde, Hydrogen Peroxide, and Methyl Peroxide in Air by Chemical Ionization Mass Spectrometry: Airborne Method

A chemical ionization mass spectrometry (CIMS) method utilizing a reagent gas mixture of O2, CO2, and CH3I in N2 is described and optimized for quantitative gas-phase measurements of hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), formic acid (HCOOH), and the sum of acetic acid (CH3COOH) and hydroxyacetaldehyde (HOCH2CHO; also known as glycolaldehyde). The instrumentation and methodology were designed for airborne in situ field measurements. The CIMS quantification of formic acid, acetic acid, and hydroxyacetaldehyde used I− cluster formation to produce and detect the ion clusters I−(HCOOH), I−(CH3COOH), and I−(HOCH2CHO), respectively. The CIMS also produced and detected I− clusters with hydrogen peroxide and methyl peroxide, I−(H2O2) and I−(CH3OOH), though the sensitivity was lower than with the O2− (CO2) and O2− ion clusters, respectively. For that reason, while the I− peroxide clusters are presented, the focus is on the organic acids. Acetic acid and hydroxyacetaldehyde were found to yield equivalent CIMS responses. They are exact isobaric compounds and indistinguishable in the CIMS used. Consequently, their combined signal is referred to as the acetic acid equivalent sum. Within the resolution of the quadrupole used in the CIMS (1m∕z), ethanol and 1- and 2-propanol were potential isobaric interferences to the measurement of formic acid and the acetic acid equivalent sum, respectively. The CIMS response to ethanol was 3.3% that of formic acid and the response to either 1- or 2-propanol was 1% of the acetic acid response; therefore, the alcohols were not considered to be significant interferences to formic acid or the acetic acid equivalent sum. The multi-reagent ion system was successfully deployed during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) in 2014. The combination of FRAPPÉ and laboratory calibrations allowed for the post-mission quantification of formic acid and the acetic acid equivalent sum observed during the Deep Convective Clouds and Chemistry Experiment in 2012

An Ion-Neutral Model to Investigate Chemical Ionization Mass Spectrometry Analysis of Atmospheric Molecules – Application to a Mixed Reagent Ion System for Hydroperoxides and Organic Acids

An ion-neutral chemical kinetic model is described and used to simulate the negative ion chemistry occurring within a mixed-reagent ion chemical ionization mass spectrometer (CIMS). The model objective was the establishment of a theoretical basis to understand ambient pressure (variable sample flow and reagent ion carrier gas flow rates), water vapor, ozone and oxides of nitrogen effects on ion cluster sensitivities for hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), formic acid (HFo) and acetic acid (HAc). The model development started with established atmospheric ion chemistry mechanisms, thermodynamic data and reaction rate coefficients. The chemical mechanism was augmented with additional reactions and their reaction rate coefficients specific to the analytes. Some existing reaction rate coefficients were modified to enable the model to match laboratory and field campaign determinations of ion cluster sensitivities as functions of CIMS sample flow rate and ambient humidity. Relative trends in predicted and observed sensitivities are compared as instrument specific factors preclude a direct calculation of instrument sensitivity as a function of sample pressure and humidity. Predicted sensitivity trends and experimental sensitivity trends suggested the model captured the reagent ion and cluster chemistry and reproduced trends in ion cluster sensitivity with sample flow and humidity observed with a CIMS instrument developed for atmospheric peroxide measurements (PCIMSs). The model was further used to investigate the potential for isobaric compounds as interferences in the measurement of the above species. For ambient O3 mixing ratios more than 50 times those of H2O2, O3−(H2O) was predicted to be a significant isobaric interference to the measurement of H2O2 using O2−(H2O2) at m∕z 66. O3 and NO give rise to species and cluster ions, CO3−(H2O) and NO3−(H2O), respectively, which interfere in the measurement of CH3OOH using O2−(CH3OOH) at m∕z 80. The CO3−(H2O) interference assumed one of its O atoms was 18O and present in the cluster in proportion to its natural abundance. The model results indicated monitoring water vapor mixing ratio, m∕z 78 for CO3−(H2O) and m∕z 98 for isotopic CO3−(H2O)2 can be used to determine when CO3−(H2O) interference is significant. Similarly, monitoring water vapor mixing ratio, m∕z 62 for NO3− and m∕z 98 for NO3−(H2O)2 can be used to determine when NO3−(H2O) interference is significant

Characterization of Unknown Chemicals Using Gas Chromatography/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and AB-Initio Calculations

The major goal of this dissertation was to combine modern mass spectral techniques and advanced ab-initio molecular orbital calculations for analytical characterization of complex sample mixtures. Multidimensional mass spectral techniques and theoretical molecular modeling approaches were developed to identify disinfection by-products (DBPs) that were present in drinking water samples from local sources. Analogous methodologies that utilized thermochemical analysis, including both experimental and theoretical calculations, were employed to improve analytical resolution and identify components of a complex petroleum mixture. In the process of developing these multidimensional methods, we were able to address challenges such as solvent contamination that are associated with identification of DBPs using conventional methods. Moreover, we were able to develop a complementary theoretical method that utilizes proton transfer (PT) energy profiles for proton affinity (PA) determinations. The use of highly accurate and precise thermochemical data such as PAs can be advantageous for unknown identification when mass spectral data and/or fragmentation patterns are not available in commercial databases. In chapter 1 of this thesis, a brief review of the experimental approaches is presented. The application of the GC/FT-ICR MS for identification of DBPs in drinking water is discussed in chapter 2; in part one of this chapter, a comprehensive literature review of the research field is presented. Part 2 of chapter 2, focuses on addressing the issues related with the solvent artifact complications and the use of solid-phase microextraction method for DBP analysis. In chapter 3, we show the importance of the determination of the experimental gas phase basicity as an additional analytical dimension for mass spectral analysis of unresolved GC peaks. In chapter 4, an overview of quantum mechanical methods for the determination of structures and energetics is provided. For example, the decomposition reaction of dichloromethanol catalyzed by a water molecule is discussed. Various important details on high accuracy ab-initio G2 and G3 calculations and the utility of the PT energy profile for PA determination of reagent and halogenated compounds are discussed in chapter 5. In chapter 6, we show the utility of combined experimental and theoretical study on potential C7H8 isomers in gasoline samples